1. Field of the Invention
This invention relates to an optoelectronic module (light emitting device or light receiving device) for allowing the module to be attached to and detached from an external optical fiber. There are two types of optical modules for coupling an optoelectronic device with an optical fiber. One is a receptacle type module. The other is a pigtail type module. The pigtail type faces an end of an intermediate fiber with an optoelectronic device in a housing and permanently fixes the fiber to the housing. The rest of the intermediate fiber projects outward from the module. The other end of the intermediate fiber is fixed in an optical connector which allows a ferrule holding an end of an external optical fiber to detachably combine with. Thus in the pigtail type module, the fiber confronting the optoelectronic device is tightly fixed to the housing of the module.
The receptacle type module couples an optical fiber with an optoelectronic device by mounting the optoelectronic device into a housing having a cylindrical sleeve therein, inserting a ferrule holding an end of an optical fiber via an end hole of the housing, facing the fiber end with the optoelectronic device, aligning the fiber end to the optoelectronic device and fixing the ferrule to the housing. A lot of receptacle type modules have been utilized. Receptacle type modules require alignment of an optoelectronic device, a lens and an optical fiber before fixation of a package including the optoelectronic device, the lens holder and the receptacle for sustaining the ferrule. All the components, the package (stem & lens holder), the sleeve and the receptacle are made of metal.
2. Description of Related Art
A module containing a laser diode has a problem. The laser diode is driven at a driving frequency fd by a driving circuit. The driving frequency fd increases with an increment of the amount of information. High driving frequency induces the module to emit radio wave of the driving frequency fd in air. Nothing serious is not caused by the radiation of the driving frequency radio wave fd from the laser module in usual cases. However, sometimes the radio wave of the driving frequency fd emitted from the laser module acts as noise and causes troubles on other electronic apparatus. Thus, noise emission from laser modules has been regulated by rules. A structural contrivance of prohibiting laser modules from emitting noise radio waves is ardently desired.
There is a standard named FCC Part 15 for regulating properties of optoelectronic modules in the United States of America. FIG. 7 demonstrates an apparatus for measuring radio wave levels at some distance induced by an object optoelectronic module for examining the properties. In an electromagnetism shielding room which inhibits external radio waves from entering, a table 43 is placed upon a reference ground 42. The object optoelectronic module 44 is laid upon the table 43. A stand 45 is laid on the reference ground at a point separated from the module by a standard distance. A horizontal arm 46 is sustained at a height by the stand 45. The height of the arm 46 is adjustable. An antenna 47 is fixed at a front end of the arm 46. The distance from the module 44 to the antenna 47 should be adjusted to a standard length (3 meters or 10 meters). The laser module is driven by a driving circuit with a driving frequency fd, a radio wave of the same frequency fd is emitted from the receptacle of module. The receptacle of the module acts as an antenna. The radio waves flying from the module are caught by the antenna 47 of the measuring apparatus. The measuring apparatus analyzes electric field intensity (dBμV/m) of radio waves per unit frequency and obtains an electric field spectrum sensed by the antenna 47 in the electromagnetism shield test chamber.
The formal standard distance between the object module and the testing antenna is stipulated to be 10 m in the US standard of FCC Part 15. The critical values which should be margins of allowable electric fields are different for frequencies. There are two kinds of standards. One is Class A. The other is Class B. Class B is more rigorous than Class A. Class B requests more sophisticated modules than Class A. The stipulated distance is 10 meters. The 10 m distance requires a very wide electromagnetism shield room. Then the critical electric field intensity is reduced from the 10 m distance values to calculate values of a shortened distance of 3 meters. The reduction from the 10 m distance to the 3 m distance can be done only by relying upon pure physics. A lower frequency range from 10 kHz to 1 MHz induces a difference of about 30 dB between the 10 m distance measurement and the 3 m distance measurement. A higher frequency range from 100 MHz to 10 GHz induces a difference of about 20 dB between the 10 m distance measurement and the 3 m distance measurement. There is a reduction table which determines the relations of allowable margins (critical values) between the 3 m case and the 10 m case. Table 1 denotes the critical values (allowable margins) of Class A and Class B from 30 MHz to 960 MHz on the 3 m distance measurement.
TABLE 1Frequency-dependent critical electric fields stipulated by FCC Part 15FrequencyLevel [dBμV/m][MHz]Class AClass B30–8849.540 88–2165443.5216–96056.946960<6054
Class B proposes more rigorous request than Class A. Thus, Class B matters for the present invention. Attention should be paid to Class B. The US standard requires electric fields less than the stipulated critical values in the whole radio wave spectrum. Background noise (natural noise) is always less than the above-listed values. What matters is the noise at the driving frequency fd at which laser diodes (LDs) or light emitting diodes (LEDs) are driven. When a driving circuit supplies an AC driving current of fd to LDs or LEDs, the driven module emits a radio wave of the driving frequency fd. The emitted radio waves are strengthened with an increase of the driving frequency fd. Here, fd=10.312 MHz is employed as an example of the driving frequency fd at which a laser diode is driven.
In the measurement, the antenna should receive radio waves ranging from 30 MHz to 18 GHz and the measuring apparatus should investigate intensity of an electric field per frequency (electric field spectrum of radio waves). The most significant matter is noise intensity at the driving frequency fd, because the driving circuit may emit a strong radio wave of the driving frequency in air via the module. The allowable upper limit in a range above 960 MHz is 54 dBμV/m in accordance with Class B of FCC Part 15. The electric field caused by optoelectronic modules should be lower than 54 dBμV/m at the driving frequency fd. However, the modules entirely built by metallic parts overreach the critical value. The whole metal modules cannot satisfy the stipulated standard of FCC Part 15, Class B. One purpose of the present invention is to provide an optoelectronic module which satisfies the above-listed standard of FCC Part 15, Class B.
No single antenna can receive such a wide range of radio waves. The measuring apparatus makes use of two kinds of antennae. 30 MHz to 1 GHz radio waves are caught by a bilog antenna. 1 GHz to 18 GHz radio waves are detected by a horn antenna. The spectrum of radio waves emitted from optical modules is measured by an apparatus having the two kinds of antennae and a spectrum analyzer in the electromagnetism shield chamber.
Japanese Utility Model Laying Open No. 4-130460, “OPTOELECTRONIC RECEPTACLE MODULE” proposed an optoelectronic module having an insulator between a package including an optoelectronic device and a receptacle holding an end of an optical fiber.
Insertion of the insulator had purposes of avoiding occurrence of troubles on the optoelectronic device at welding and enabling the module to be applied to both a +5V power source and a −5V power source.
FIG. 6 denotes a sectional view of the proposed module. The module includes a cylindrical receptacle 60 which allows an optical fiber to be attached to or detached from, a cylindrical holder 58 welded to the receptacle 60, a package (55, 56, 57) welded to the holder 58, and an optoelectronic device 50 supported in the package (55, 56, 57). What is important is that the package is divided into three parts (55, 56, 57) in the vertical direction. An upper metal ring 57 and a lower metal ring 55 sandwich a middle insulating ring 56 in the package. An adhesive glues the insulating ring 56 to both the upper metal ring 57 and the lower metal ring 55.
The insulating ring 56 divides the package into two metallic parts 55 and 57. The lower metal ring 55 sustains the optoelectronic device 50. The upper metal ring 57 connects the package to the cylindrical holder 58 and the columnar receptacle 60.
Thus the insulating ring 56 electrically separates the optoelectronic device 50 from the receptacle 60. The receptacle 60 has axial holes 66 and 65 for attaching a ferrule holding an end of an optical fiber. A narrower hole 64 follows the axial holes 65 and 66. A cylindrical lens 63 is fixed at an end of the series of axial holes for converging signal light on the fiber end or the optoelectronic device.
Japanese Utility Model Laying Open No. 4-130460 aimed at avoiding troubles on optoelectronic device induced by welding of the receptacle on the package and caused by contact with other metallic parts at a high voltage. Japanese Utility Module Laying Open No. 4-130460 is indifferent to the stipulated standard of FCC Part 15. No document other than this has found out. Thus Japanese Utility Module Laying Open No. 4-130460 has been cited here.
Japanese Utility Model Laying Open No. 4-130460 electrically separates the receptacle 60 from the optoelectronic device 50 by inserting the insulator 56 midway in the package of the module. The purpose of the interposition of the insulator is to separate the optoelectronic device from the voltage of the receptacle 60, to prevent welding from inducing troubles in the optoelectronic device, to allow both a +5V power source and a −5V power source to drive the optoelectronic module, to suppress shortcutting by a contact with other devices, and to prevent troubles from occurring on operation.
Japanese Utility Model Laying Open No. 4-130460 is different from the present invention in purposes. The present invention aims at suppressing a module from emitting electromagnetic noise to surroundings. Japanese Utility Model Laying Open No. 4-130460 is indifferent to radio noise pollution induced by its own module. Japanese Utility Model Laying Open No. 4-130460 and the present invention, however, have a common contrivance of sandwiching an insulator by the receptacle and the optoelectronic device. The prior art reference is cited now for the reason. No module proposed by Japanese Utility Model Laying Open No. 4-130460 has been produced. Perhaps the assumed necessity of driving the module by binary power sources of +5V and −5V has never been urgent till now.
No actual product has been yet produced by teaching of Japanese Utility Model Laying Open No. 4-130460. Nobody has been aware of actual drawbacks of Japanese Utility Model Laying Open No. 4-130460. The lower package part 55 carrying the optoelectronic chip, the insulating ring 56 and the upper package part 57 are glued at sticking planes 68 and 69 by an adhesive. The holder 58 is welded upon the upper package part 57 at a junction surface 59. The receptacle 60 is welded to the holder at a junction plane 62. The parts are aligned by introducing a light beam along an axial line and are fixed at optimum relative positions. A high degree of freedom of combination in the horizontal direction is an advantage.
On the contrary, Japanese Utility Model Laying Open No. 4-130460 has difficulties;    (1) rigorous precision of assembling package parts,    (2) unstable temperature dependence, and    (3) low reliability.
The above difficulties are induced by the following reasons. Thermal expansion coefficients are quite different for the upper/lower metal parts 55/57 and the insulator 56. Changes of temperature cause expansion or contraction of the insulating part 56 and the metal parts 57/55. The degrees of expansion and contraction are different. Difference of deformation induces axis misalignment and peeling of an adhesive. Neither welding nor soldering is competent for coupling an insulator to metal parts. Japanese Utility Model Laying Open No. 4-130460 glues the insulator to the metal parts with the adhesive. But the adhesive has poor reliability. Shear stress induced by thermal expansion or shrinkage would peel the adhesive.
It is unclear whether a structure of the module made in accordance with the teaching of Japanese Utility Model Laying Open No. 4-130460 satisfies the regulated standards stipulated by FCC Part 15.